1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
Conventionally, in an image forming apparatus such as a photocopier, a printer, and a facsimile in an electrophotographic recording system, a photoreceptor drum having a photosensitive layer on a surface thereof is used, an electrostatic latent image is formed by exposing a charged surface of the photoreceptor drum to signal light according to signal information to form an electrostatic latent image, a toner image is formed by supplying a toner and developing the electrostatic latent image, the toner image is transferred onto a recording medium, and then the toner image is fixed to the recording medium, whereby an image is formed on the recording medium. In the electrophotographic recording system, a toner used for forming the image on the recording medium is generally a granular substance obtained by dispersing a colorant, a release agent, a charge control agent, and the like in binder resins and finely granulating the resultant product. A heat fixing method, a wet fixing method, and the like are used for fixing the toner to the recording medium. Among these methods, since the heat fixing method is simple in configuration of an apparatus and easily and reliably fixes the toner image to the recording medium, the heat fixing method is mostly used in recent.
According to heat fixing method, for example, there is used a fixing section including a fixing roller heated at a temperature at which the toner is molten and a pressing roller provided so as to press the fixing roller and to be joined thereto. The recording medium on which a toner image is transferred (hereinafter referred to as “toner image transferred recording medium”) passes through a pressure-contact portion (fixing nip portion) between the fixing roller and the pressing roller and thus the toner image is fixed to the recording medium. The heat fixing method is mainstream in the recent image forming apparatus. However, since the binder resin of the toner should be heated at a high temperature of 100° C. or more capable of being softened or molten, power consumption is large and it may be necessary to be improved in a point of view of energy-saving. In addition, when the fixing roller does not rise to a predetermined temperature, the fixation is not preformed. Accordingly, a time arriving at a predetermined temperature, that is, a warm-up time, is required.
Further, in order to fix a multicolored toner image to the recording medium, more time is required than a single colored toner image. Since the inside of the image forming apparatus is at a high temperature and thus heat resistance of a constituent provided in the inside of the image forming apparatus and heat insulating property should increase, the material cost increases. In addition, a higher output heating section is required than a heating section in the past in accompaniment with a high speed of the image forming speed in recent. However, when the output of the heating section becomes higher, the image forming apparatus becomes larger. This is not preferable in term of the present situation where the image forming apparatus becomes smaller.
Meanwhile, in the wet fixing method, a fixing fluid including water and liquid which is soluble and dispersible in water and which softens and swells the toner are used so as to fix the toner image to the recording medium. According to the wet fixing method, the toner constituting the toner image is softened or swelled by applying the fixing fluid to the toner image and the toner image is fixed to the recording medium by transferring the toner image formed of the softened or swelled toner to the recording medium under pressure. Since power consumption in the wet fixing method is very lower than that in the heat fixing method, the wet fixing method is a useful method in term of energy-saving. Also, in term of fixing time of the multicolored toner image, since a great amount of heat is not required, the time can be shortened more than the heat fixing method. Accordingly, by using the wet fixing method with the heat fixing method, it has been proposed the image forming speed becomes higher without decrease in consumption and increase in size of the image forming apparatus.
For example, there is provided an image forming apparatus including a toner image forming section, a transfer section, a fixing section, a conveying section, and a fixing fluid applying section. Herein, the toner image forming section forms the toner image according to image information.
The transfer section transfers the toner image formed by the toner image forming section to the recording medium. The fixing section fixes the toner image to the recording medium by heating and pressing the toner image transferred recording medium. The conveying section conveys the toner image transferred recording medium to the fixing section. The fixing fluid applying section applies the fixing fluid to the unfixed toner image while the toner image is fixed after the transfer, thereby softening and/or swelling the toner constituting the toner image (for example, refer to Japanese Unexamined Patent Publication JP-A 2004-284306). In an image forming apparatus of JP-A2004-284306, a fixing fluid section applies a fixing fluid to a toner image before fixation and softens and swells the toner image. Accordingly, heating temperature at the time of fixing is decreased, power consumption is decreased, and thus an image forming speed may become higher.
In the image forming apparatus suggested in JP-A 2004-284306, the fixing process is hardly performed with the toner image transferred to the recording medium and the fixing fluid is applied in non-contact to the toner image which is in a state without adhesive power to the recording medium using an electrostatic ink jet recording head or the like. In this configuration, bleeding of the toner, aggregation of the toner, and unevenness of a toner image caused thereby when droplets of the fixing fluid are fixed to the toner image. Accordingly, image reproducibility and image quality may decrease. For this reason, in the image forming apparatus of JP-A 2004-284306, it is proposed that the toner image transferred to the recording medium is once heated, the toner constituting the toner image is molten and fixed to the recording medium to some extent, and then the fixing fluid is applied. In this case, since the toner should be molten and fixed to the recording medium under non-pressure, the toner should be heated at a very high temperature for melting and fixing the toner. The heating temperature is influenced by conditions such as transfer speed. However, for example, when the transfer speed is 200 mm/sec in line, a temperature in the range of 140 to 160° C. is needed. Accordingly, in the image forming apparatus of JP-A2004-284306, it is difficult to extremely reduce power consumption. In addition, when the fixing fluid is applied to the toner image on the recording medium, the recording medium absorbs the fixing fluid, whereby curl and creases may occur.